Chronic airway diseases are characterized by alterations in the cellular composition of the tracheobronchial epithelium. Depletion of cells within the secretory/ciliated cell lineage and hyperplasia of cells within the basal cell lineage suggests that cell fate decisions at the level of a common precursor cell are dysregulated. The essential role played by the Wnt pathway in tracheobronchial development and reactivation of this pathway in regions of basal cell hyperplasia identify the Wnt signal transduction pathway as a potential regulator of tracheobronchial precursor cell differentiation. However, the complex pathology of end stage human lung disease prohibits identification of the source and target of Wnt signals. To this end, we have focused on developing genetically modified murine models that, in combination with quantitative histopathology, allow us to investigate the nature of precursor-progeny relationships in the epithelium and critical elements of Wnt signaling in the airway of intact mice. Previous studies have demonstrated that the tracheobronchial and bronchiolar regions are maintained by divergent stem cell hierarchies and suggest that the Wnt signal transduction pathway regulates distinct aspects of homeostasis and repair that may be critical to establishment and maintenance of structural and functional distinctions within these regions. Lineage tagging analysis has allowed us to conclude that the K14EC functions as the effecter cell in tracheobronchial regeneration and identification of multi-, uni-, and bi-potential K14EC-derived clones suggest that regulative processes influence differentiation of the K14EC. Preliminary studies identified p-catenin as a potential regulator of K14EC fate and the canonical Wnt signaling pathway as the proximal signaling mechanism leading to stabilization of p-catenin. These observations support the hypothesis that the tracheobronchial epithelium is maintained by a classical stem cell hierarchy and that cell fate decisions within this hierarchy are regulated by the canonical Wnt/p-catenin signal transduction pathway. Specific Aims will test this hypothesis in vivo through genetic manipulation of components of the canonical Wnt/p-catenin pathway and subsequent quantitative assessment of proliferation and differentiation. These studies are designed to identify mechanisms regulating the tracheobronchial stem cell hierarchy. Successful completion of this project will yield insights into critical transitions that are susceptible to dysregulation in human lung disease.